Such long distance hydrocarbon transfer systems for near surface transfer of fluids from a first floating or fixed structure to a second floating structure are known from WO 2005/090152 and from SBM Offshore Annual Report 2005, and may be used in an offshore field development that is based on for example a Floating Production Storage and Offloading unit (FPSO) and a wellhead Spar Dry Tree Unit (DTU). Hydrocarbon fluids are transported in flow lines in a Gravity Actuated Pipe (GAP™) or midwater pipe system from one floating structure to the other. A bundle of flow lines is supported along a frame in a substantially horizontal direction. Due to thermal expansion an contraction of the flow lines and due to expansion in view of loading variations, stresses may occur in these flow lines, resulting in a reduced useful service life of the known transfer system. Furthermore, the motion of the floating structures may be transferred to the end parts of the support frame and to the flow line bundles, which may cause adverse fatigue effects.
Environmental conditions, distance between the floating structures, the nature of floating structures, the number of flow lines, fluid properties, fabrication/launch site characteristics and tow-out distance are the main design parameters for the known midwater pipe system. The midwater pipe system fatigue life is a very important design aspect as it comprises uneven contributions from launching, towing, installation and in-place service.
It is known to connect two floating offshore structures via a midwater pipe system for conveying hydrocarbons from one structure to the other. One floating structure may be a production or storage structure such as a spar buoy, a semi-submersible structure, a fixed tower or a mooring buoy whereas the second structure may comprise a floating production storage and offloading vessel (FPSO), a shuttle tanker and the like.
Such a known system is described in U.S. Pat. No. 6,394,154 in the name of the applicant. The transfer system includes two generally vertically oriented duct sections which are placed at an angle with the vertical. These two sections are connected to a substantially horizontal third member, for instance a third duct section. Near the connection points of the vertically oriented duct sections and the horizontal member, a tensioning weight is provided such that a tensioning force in the horizontal duct section is created. Hereby bending/kinking and/or buckling due to currents or floating systems dynamics is reduced. A relatively long horizontal duct section can be used which is preferably made of hard pipe such as rigid steel pipe.
The international patent application WO2006/120351 relates to a device for transporting fluid between two floating support structures, each anchored to the sea bed, comprising an submerged central rigid pipeline each end of which is connected to a respective floating support structure by means of a flexible pipeline. Due to its curved configuration, the rigid pipeline is not situated in a horizontal plane between its two extremities. One end is connected by a tensioning cable to a floating support structure without being connected to the sea bed and cooperates with ballasting means while the other extremity is connected to the sea bed via a chain while interacting with buoyancy means at the water surface.
U.S. Pat. No. 6,769,376 discloses systems and methods for prevention of clashing between multiple steel pipes spaced closely together and to methods of installation of multiple pipes at the same time. The system separates the transfer conduits and allows for a relative motion between the transfer conduits.
The transfer system is directly connected to the floating structures and forms a U-shape. No axial tension members are connected to the ends of the flow line.